<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            background-color: aqua;
            width: 500px;
            height: 500px;
            position: absolute;
            margin: auto;
            top: 0;
            bottom: 0;
            left: 0;
            right: 0;
        }
        /* .son{
            width: 100px;
            height: 100px;
            background-color: blue;
           
            
            top: 0;
            bottom: 0;
            left: 0;
            right: 0;

        } */
    </style>
</head>
<body>
    <div class="box">
        <div class="son"></div>
    </div>
</body>
</html>